The invention relates to a burner having a combustion-air feed duct, and more specifically a burner for use in turbine engines.
The present invention relates to the area of cumbustion in turbine engines. The book entitled xe2x80x9cBerechnung der Schallausbreitung in durchstromten Kanalen von Turbomaschinen unter besonderer Berxc3xccksichtigung der Auslegung von Drehtonschalternxe2x80x9d [xe2x80x9cCalculation of the sound propagation in flow ducts of turbomachines, taking particular account of the design of rotational sound switchesxe2x80x9d] Section 3.4, by Christian Faber, Verlag Shaker, Aachen 1993, illustrates how discontinuities in flow ducts influence the propagation of sound in a fluid flowing in these flow ducts. Scatter, reflection and transmission factors are derived, by means of which it is possible to calculate which part of incident sound energy passes the discontinuity and which part is reflected.
Another reference is German Patent No, DE 44 30 697 C1 which shows an incoming-air sound absorber. The incoming-air sound absorber comprises a flow line which is surrounded by an impervious wall and through which a gaseous medium flows at subsonic speed. A device for suppressing airborne sound emissions is arranged in the flow line. As seen in the direction of flow of the medium, this device is arranged upstream of a sound-emitting noise source and is used to suppress the emissions of airborne sound in the opposite direction to the direction of flow. The device has a constriction, which is similar to a laval nozzle, in the flow line. This constriction, in the form of a laval nozzle, accelerates the velocity of the gaseous medium to the speed of sound. This builds up a reflection barrier to the airborne sound.
Combustion oscillations may occur in combustion systems. Combustion oscillations of this type are described in the article xe2x80x9cCombustion-Driven-Oscillations in Industryxe2x80x9d by Abbott A. Putnam, American Elsevier, New York 1971. In accordance with the Rayleigh criterion, a combustion oscillation is built up when heat is periodically supplied to a quantity of air in a combustion chamber. This supply of heat takes place as a periodic combustion output release in phase with a characteristic oscillation of the air in the combustion chamber. Accordingly, the combustion oscillation can be suppressed by a release of power of the opposite phase. Combustion oscillations of this type may lead to considerable noise pollution and even to mechanical damage to components of the combustion device. It is stated in the above article, on page 4 under the paragraph xe2x80x9cPulsations in supply ratexe2x80x9d that the combustion oscillation may be coupled to an air or fuel supply. To avoid the propagation of pulsations in the supply systems, it has been proposed to bring about a considerable pressure loss in the supply systems, in order in this way to construct a reflection barrier. However, it has already been pointed out that a pressure loss of this type is generally unacceptable.
In the article entitled xe2x80x9cMaxcex2nahmen zur Vermeidung von Verbrennungsschwingungenxe2x80x94Kennzahl zur strxc3x6mungsakustischen Entkopplung am Brennerxe2x80x9d [Measures aimed at avoiding combustion oscillationsxe2x80x94characteristic variable for flow-acoustic decoupling at the burnerxe2x80x9d] by D. Schrxc3x6der, Gaswxc3xa4rme International, Vol. 41, section 1, January 1992, D. Schrxc3x6der has developed a flow-acoustic limit value criterion for decoupling a combustion chamber from a coupled system of pipes. The decoupling is effected by a reflection area, which is produced in particular at the burner by means of a narrowing of the cross section of a feed pipe and, if appropriate, in addition by a perforated plate arranged at this cross-sectional narrowing. However, these measures have the drawback of a considerable pressure loss for the medium which is fed to the burner.
It is an object of the present invention to provide a burner in which a combustion zone into which the burner opens out is decoupled from a feed line for combustion air for the burner in terms of flow acoustics, this decoupling at most resulting in an acceptable additional pressure loss in the combustion air.
According to the present invention, one object is achieved by a burner having a combustion air duct, in which a swirl generator, which is formed from a number of swirl-generator elements, is arranged in such a way that the swirl generator increases the mean velocity at which the combustion air passes through the swirl generator to a Mach number of at least 0.4, in particular at least 0.6. The mean velocity of flow in this context is the mean formed for the velocity over a cross section of the combustion air duct.
Swirl generators are often used in a burner to impart a swirl, which stabilizes the combustion flame, to the combustion air entering the combustion chamber. A reflection barrier for sound waves is built up using the swirl generators by means of simultaneous acceleration of the combustion air by means of the swirl generators to a Mach number of at least 0.4. This weakens or even suppresses the propagation of combustion oscillations into the feed line system for combustion air. By building up the reflection barrier by means of the swirl generator, a pressure loss in the combustion air can be kept at a low level. Therefore, the acoustic decoupling has at most a slight negative effect on the efficiency of a combustion device in which the burner is integrated.
It is preferable for a swirl-blade ring comprising swirl blades for imparting a swirl to the combustion air to be arranged in the combustion air duct. It is also preferable for the swirl generator to be formed by the swirl-blade ring. Therefore, instead of providing additional swirl generators for acoustic decoupling, a swirl-blade ring which is present in any case is designed as an acoustically decoupling swirl generator. Designing the swirl-generating elements as swirl blades results in a measure which is easy to implement in order to keep the pressure loss in the combustion air at a low level. This is because acceleration of the combustion air when it enters the swirl-blade ring as a result of an effective narrowing of the cross section is followed again by a widening, by means of which pressure is recovered in the combustion air, on account of the blade profiles which narrow in the direction of flow. Therefore, designing the flow generator as a swirl-blade ring has the advantage both that a means which is already present is provided for generating a combustion-stabilizing swirl, and that pressure recovery, which has a favorable effect on efficiency, becomes possible in the combustion air.
The swirl-blade ring preferably has first and second blades which alternate with one another over the circumferential direction of the swirl-blade ring, the second blades being offset with respect to the first blades in the opposite direction to a direction of flow of the combustion air. The first blades preferably have a first maximum profile thickness and the second blades preferably have a second maximum profile thickness, the first maximum profile thickness being greater than the second maximum profile thickness.
The first blades have a first chord length and the second blades have a second chord length. In this context, the first chord length is preferably shorter than the second chord length. The swirl generator is therefore formed to a certain extent from two partial blade rings which engage in one another in an offset manner in the direction of flow. The blades of one of the partial rings are preferably longer and thinner than the blades of the other partial ring, and specifically it is preferable for the blades of that partial ring which is arranged in front of the other partial ring, as seen in the direction of flow, to be longer and thinner. This design enables the two methods of operation of the swirl-blade ring to be optimized, i.e. both the function of swirl generation and the function of acoustic decoupling can be fulfilled to a sufficient extent by suitable dimensioning and matching of the partial rings to one another.
Furthermore, this structure results in a simple way of retrofitting a swirl-blade ring in a burner in such a way that it subsequently allows the desired acoustic decoupling. For this purpose, it is simply necessary for a further swirl-blade ring to be inserted into the existing swirl-blade ring. This is achieved by arranging an additional swirl blade between in each case two existing swirl blades. Suitable dimensioning of the additional swirl blades results in the desired acceleration of the combustion air to a Mach number of over 0.4, preferably over 0.6, more preferably over 0.8. At the same time, the profile of the additional swirl blades is designed in such a way that a recovery of pressure is achieved in the combustion air. This is preferably achieved by means of a gradually widening passage cross section. In particular, this gradual widening is to be designed in such a way that there is no flow separation along the swirl blades.
The combustion air duct is preferably of annular design. Preferably, fuel can be admitted to the combustion air duct, and in the process this fuel is intensively mixed with the combustion air prior to combustion. Furthermore, it is preferable for it to be possible for the fuel to be admitted from at least some of the swirl-generating elements. The intensive mixing of the fuel with the combustion air prior to combustion (premix burner) leads to a reduction in the emissions of nitrogen oxides. This is achieved by making the flame temperature more uniform on account of intimate mixing, since the emissions of nitrogen oxides rises exponentially with the flame temperature. A further advantage of the acoustic decoupling by means of the swirl generator is additional mixing of fuel and combustion air, since, on account of the pronounced acceleration of the combustion air and of the adjoining zone of pressure recovery, additional turbulence in the combustion air leads to a further improvement in the mixing of combustion air and fuel. If appropriate, the swirl generator may also be dimensioned in such a way that some of the pressure recovery is dispensed with in favor of mixing which is improved by increased turbulence.
The burner preferably has an additional pilot burner, which is used to stabilize combustion of the fuel/combustion air mixture emerging from the combustion air duct. If the pilot burner operates as a diffusion burner, i.e. fuel and combustion air in the pilot burner are only mixed at the location of combustion, the burner is also known as a hybrid burner, in which both premix combustion and diffusion combustion takes place.
The burner is preferably designed as a gas turbine burner. Particularly in the case of a high power conversion of a gas turbine, combustion oscillations with very high amplitudes and possibly considerable damaging effects may occur. The flow-acoustic decoupling from the combustion-air supply system is of particular importance in this context. This applies in particular to stationary gas turbines.